1. Field of the Invention
The problem being addressed by this disclosure is that of reducing the cost of fabricating a high-performance thin-film solar cell. In particular, a low-cost solution-based process for depositing the chalcogenide-based absorber layer in a thin-film solar cell is disclosed.
2. Description of the Related Art
World demand for energy is expected to more than double by 2050 and to more than triple by the end of the century. Incremental improvements in existing energy technologies will not be adequate to supply this demand in a sustainable way. Sunlight represents by far the largest of all carbon-neutral potential energy sources. More energy from sunlight strikes the earth in one hour than all the energy consumed on the planet in one year. However, currently, less than 0.1% of the world's electricity is generated using solar electricity generation (see, for example, Basic Research Needs for Solar Energy Utilization, Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, Apr. 18-21, 2005, US Department of Energy, Office of Science).
The reason for the lack of widespread adoption of PV technology is directly linked with the high cost/watt of this technology. Industrial electricity today costs, on average, about 0.06$/kWh in the United States. Solar electricity costs vary between approximately 0.2-0.8 $/kWh depending upon the system and climate. To be competitive without subsidies, photovoltaics must achieve an at least a 5-10× reduction in the per kWh cost.
Solar cells incur no fuel expense, but they do have significant capital cost. The cost for the electricity produced by a solar cell is calculated by amortizing the capital cost over the lifetime of the cell (about 30 years) and considering the total electrical output energy produced over the cell lifetime. The cost figure of merit for PV cell modules ($/Wp) is determined by the ratio of the module cost per unit area ($/m2) divided by the maximum amount of electric power delivered per unit of cell area (Wp/m2). Consequently, to significantly reduce the cost of PV technology, it is necessary both to maintain/increase cell efficiency (enabling higher WP/m2), as well as to significantly reduce the costs of PV module fabrication and installation (see, for example, Basic Research Needs for Solar Energy Utilization, Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, Apr. 18-21, 2005, U.S. Department of Energy, Office of Science).
More than 90% of today's PV production is Si-based. High efficiencies (>20% in laboratory-scale devices) are only achieved for single junction cells in thick crystalline silicon devices. Due to the indirect band gap, which necessitates the thick absorber layer, and the associated high-temperature vacuum-based processing, Si is not an ideal material for an absorber layer. An alternative approach is to look at thin-film direct band gap absorber layers (rather than Si), which are typically metal chalcogenides offering a very high absorptivity for solar photons.
The two principal metal-chalcogenide-based thin-film technologies are CdTe and Cu(In,Ga)Se2 (CIGS). CdTe cells with 16.5% efficiency and CIGS cells with 19.5% efficiency have been made. For comparison, the efficiency of amorphous Si thin-film devices has leveled out at 12% after more than 25 yrs of research and appears to be unlikely to improve substantially any time soon. Given efficiencies approaching 20% and acceptable environmental stability characteristics without using toxic Cd, increasing emphasis is being focused on CIGS-based devices.
FIG. 1 shows a typical CIGS device structure using a glass substrate. The layers and thicknesses in the device are schematically shown. This particular structure of the CIGS-based device known in the art has a substrate (e.g., glass plate, flexible foil, etc.), a molybdenum back contact, the CIGS absorber layer, a thin buffer layer (typically CdS or Cd1-xZnxS), a bilayer of intrinsic and aluminum-doped zinc oxide or indium tin oxide, also known as ITO, as a transparent conductive oxide (TCO) front contact. Finally a metal grid is often used on top to help collect the current generated by the cell. One of the most critical layers, which also is often the thickest layer, is the CIGS absorber layer. Therefore how this layer is deposited can have a big impact on production costs and device efficiency.
FIG. 2 shows a typical CdTe device structure using a glass superstrate. The layers and thicknesses in the device are schematically shown. This is an alternative device structure, typically used for CdTe devices (but it is by no means restricted to this material).
Since the glass forms the top of the device, through which the sun penetrates into the cell, as opposed to forming the base of the structure, this configuration is known as a “superstrate” configuration and that in FIG. 1 is a “substrate” configuration.
An advantage of the substrate configuration in FIG. 1 is that the substrate does not need to be optically transparent and therefore a wider range of rigid and flexible materials can act as the substrate. In the case of the superstrate configuration, the optical quality and transparency of the superstrate can significantly impact the efficiency of the device since it can absorb the incident radiation.
Unfortunately, despite the potential for high-efficiency device performance, CIGS is a relatively complicated chemical system, having 4 or more elements (it is often doped with Na and/or S), with the device performance strongly being effected by the detailed chemical composition and compositional depth profile in the device. Since grain boundaries can act as recombination centers for the electrons and holes created during the absorption of a photon (which therefore reduces the device efficiency), it is desirable to deposit films with large (μm-scaled) grains with well-controlled stoichiometry. The CIGS material can accommodate a wide range of variation in chemical composition (e.g., variation in Ga content, Cu:(In,Ga) ratio, etc.), which can also have a substantial effect on device performance. It is therefore a significant challenge to develop techniques for reproducible deposition of thick, i.e., μm scale, well-crystallized layers of this material to act as an absorber layer.
Two multi-step vacuum-based processes represent the principal techniques being used to deposit CIGS films for the highest efficiency devices. One of the most successful techniques in terms of generating high-efficiency is “three-stage co-evaporation” from individual elemental sources in the presence of Se vapor.
The NREL (National Renewable Energy Laboratory) three-stage process for depositing CIGS includes the steps of:
(1) depositing (In,Ga)2Se3 at lower substrate temperature (from about 300° C. to about 350° C.); followed by
(2) evaporating Cu and Se at a higher temperature (from about 500° C. to about 560° C.) to yield Cu-rich CIGS; and
(3) adding more (In,Ga)2Se3, so that a slightly Cu-deficient final film composition is achieved. A Se vapor treatment is carried out during the cooling step. The Ga/(Ga+In) ratio is typically varied as a function of depth in the film in order to achieve a graded band gap that improves the separation of the photogenerated charge carriers and reduces recombination at the back contact.
Despite the success of this approach in generating high-efficiency small area devices (>19% efficiency), co-evaporation requires strict control over the evaporation fluxes in order to achieve the desired film characteristics, which can be difficult to achieve over large substrate areas. Large-area devices therefore generally have significantly lower efficiencies than for the laboratory-scale devices. In addition, much of the evaporated CIGS material does not end up on the substrate, but rather on the vacuum chamber walls. Thus, the low materials utilization adds to the cost of manufacturing prospective devices.
An alternative vacuum-based deposition approach involves selenization of metallic precursor layers. The stacked metal or alloy layers can be deposited by sputtering or evaporation methods. The incorporation of Se into the films is most commonly carried out under a Se-containing atmosphere at high temperatures, typically significantly above 400° C. H2Se is one of the most efficient selenization sources. However, it is also very toxic. In addition, the “selenization of metallic precursor layers” approach, being also a multi-step process, requires the use of high temperatures, vacuum chambers, and pumps, which limits its applicability to very large area and flexible plastic substrate applications.
Although reasonably low-cost CIGS solar modules can be produced using the vacuum-based deposition techniques described above, high initial capital investment is necessary to obtain the required sophisticated vacuum deposition equipment and thermal processing equipment with toxic gas handling capability. However, the overall economics will ultimately depend on the production volume and device efficiency. In addition, while deposition on glass can be readily achieved, the use of low-cost and flexible plastics as a substrate is generally not possible due to the high processing temperatures required in the deposition processes (for example, see M. Kernell et al., Critical Reviews in Solid State and Materials Sciences, vol. 30, 1-31, 2005; and M. Kaelin et al., Solar Energy, vol. 77, 749-756, 2004).
There are two general approaches for solution-based CIGS deposition (see M. Kaelin et al., Solar Energy, vol. 77, 749-756, 2004). In the first approach, precursors are decomposed during the solution-deposition on a substrate to form CIGS directly during the deposition. Examples of this approach include spray pyrolysis and direct electrodeposition of CIGS. The second approach involves depositing a precursor material on the substrate, with a subsequent chemical treatment and/or selenization process, often at high temperatures. Examples of this approach include electrodeposition of metal layers and nanoparticle precursor approaches.
As an example, spray pyrolysis and spray CVD, which employ organometallic precursors that are pyrolized on a substrate, have been explored for many years, but have generally resulted in CIGS devices with low device efficiency (<5%). In addition, these approaches generally require a high temperature post-deposition heat treatment to remove precursor impurities and improve crystallinity (for example, see M. Kaelin et al., Solar Energy, vol. 77, 749-756, 2004; S. Duchemin et al., Proceedings of the 9th EPVSEC, Freiburg, Germany, 476-479, 1989; and J. D. Harris et al., Mater. Sci. Eng. B., vol. 98, 150-155, 2003).
Electrodeposition is an alternative approach to directly depositing CIGS on a conducting substrate (see, for example, D. Guimard et al., Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, May 21-24, 2002, 692-695; and M. E. Calixto et al., Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Lake Buena Vista, Fla., Jan. 3-7, 2005, 378-381).
However, the above approaches generally require a high-temperature post-deposition heat treatment (generally in a Se-containing atmosphere) and have typically yielded quite low efficiency devices (<10%), unless post-deposition vacuum evaporation processes are employed to correct the film compositional makeup (see R. N. Bhattacharya et al., Thin Solid Films, vol. 361-362, 396-399, 2000). In addition, the process of electrochemical deposition is not as high-throughput as selected other solution-based processes, such as, spin-coating, doctor blading and printing.
Electrodeposition of the metals, followed by a post deposition Se treatment, and high-temperature anneal, have also been demonstrated yielding device efficiencies below 10% (A. Kampmann et al., Mat Res. Soc. Sympos. Proc., vol. 763, B8.5.1-B8.5.6, 2003).
Nanoparticle precursor approaches, such as those employed by ISET (International Solar Electric Technology, Inc.) and Nanosolar, two start-up companies focusing on the solution-deposition of CIGS, rely on forming a suspension of Cu, In and Ga oxide or metal nanoparticles in a solution, which can then be coated onto a Mo-coated surface during the device fabrication. If an oxide precursor is used, the dried precursor is first reduced under a hydrogen atmosphere at high temperature to form a metal-alloy layer. This alloy layer is then annealed under H2Se (again at high temperature) to form the CIGS absorber layer. Solar cells based on this approach have yielded efficiencies as high as 13.7% on glass, 13% on Mo foil, and 8.9% on polyimide foil. The disadvantages of this approach include potential incorporation of carbon impurities from the nanoparticle capping layer, which limits achievable efficiency.
These processes also generally employ multiple high-temperature steps, as well as the handling of toxic selenium sources in the vapor phase (e.g., H2Se). Starting from the nanoparticle precursors also renders it difficult to achieve sufficient grain growth and phase homogeneity during the deposition process, without a high-temperature thermal treatment (see V. K. Kapur et al., Proceedings of the DOE Solar Program Review Meeting (DOE/GO-102005-2067), 135-135, 2004; and C. Eberspacher et al., Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, May 21-24, 2002, 684-687).
Thin films of CIS (CIGS without the Ga) have also been prepared by spin-coating using organic-based Cu—In naphthenates as soluble precursors. The process requires a subsequent high-temperature selenization reaction in vacuum-sealed ampoules. So far, no devices have been demonstrated using this approach (see S. Merdes et al., Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden, Germany, Sep. 4-8, 2006, 1870-1873).
The references/patents found relating to solution-based processing of CIGS absorber layers generally refer to:
(1) nanoparticle approaches to deposition, whereas the present approach is a molecular precursor approach;
(2) precursor approaches that lead to oxide, nitrate, and related materials which need to be converted into chalcogenide materials using a post deposition anneal, in the presence of a chalcogen-containing gas.
Thus, the disadvantages of the known solution-based film processing approaches include:
(1) difficulty in producing films with sufficiently low levels of impurities. The lack of a vacuum environment during processing and presence of foreign elements in the starting solution necessitate a careful selection of precursors to avoid film contamination;
(2) difficulty in achieving phase purity, compositional control, and grading throughout the film thickness, in particular with respect to Ga concentration;
(3) difficulty in reducing the high temperatures and simplifying the multistep processing that is required for the solution-based approaches; and
(4) difficulty in controlling the grain structure in solution-processed films. Poor grain structure severely limits device efficiency.
Given these disadvantages of the presently known processes for solution-depositing CIGS absorber layers, it is desirable to develop devices in which high quality CIGS layers can be reproducibly deposited using a simpler, cleaner, lower-cost and higher throughput process which can overcome these disadvantages.
Accordingly, the present invention provides such a process, which produces devices having high quality CIGS layers.